The "free area" of a louver is defined by the Air Movement and Control Association (AMCA) in AMCA Standard 500, "Test Methods for Louvers, Dampers and Shutters," as "the minimum area through which air can pass" and is determined by multiplying the sum of the minimum distances between intermediate blades, top blade and head, and bottom blade and sill by the minimum distance between jambs. The percent free area is the free area divided by the gross area .times.100. The distances, in turn, are between points on the adjacent members (blades, sill and head, as the case may be) that are closest to each other in any direction. To minimize the size of the opening at the building face and the size and cost of the louver for a given air-flow capacity, it is desirable to design the louver to have a maximum free area. Because of irregularities, such as drainage troughs, offsets, flanges, screw bosses and the like, in the blade cross sections, few, if any, louvers currently on the market have a maximum free area.